Since July 2014, Tian-Guang HVDC project happened the events of the converter valve abnormal breakover four times, which caused the main equipment overcurrent or overvoltage and seriously influenced the safe and stable operation of Tian-Guang HVDC project. In order to identify the cause of the converter valve abnormal breakover, the theoretical analysis, field checking, simulation analysis and physical test are done from the aspects of the control system, the VBE system, the converter valve and its components and the TE boards, which successfully eliminates the possibility of the converter valve abnormal breakover by the control system, VBE system, converter valve and its components. Through the analysis and high voltage test of the BTC circuit of the TE boards, it is found that the TE boards will mis-trigger the thyristor under the high pollution and humidity condition. Based on the analysis of the relationship of the BTC acting threshold and the number of the abnormal TE boards, the decrease of the threshold of the partial BTC is set in the EMTDC simulation on the four events, and the simulation waves are highly similar to the field fault waves. The results show that even if part of TE boards are clean and the rest are polluted seriously, the TE boards also would lead to the abnormal breakover of the converter valve in the humid environment.